Blackbird
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J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Sat Dec 29, 2007 4:51 pm

Okay... the JT8D's spool up time is around 6.5 to 8 seconds. What was the original J-52's spool-up rate? -- It seemed as if the A-6 intruders spooled up faster than that?

Also the RM-8, was it modified at all from the baseline JT8D design? Was it's spool-up rate different? Was it designed to fly faster?

(Assuming none of this is classified -- I doubt it would be since the A-6 is an outdated aircraft -- that is)


I'm not asking this question on the military forum because the JT8D was used for both civilian and military uses and I figure should be under tech-ops which is not military (or even civilian) specific.


Andrea Kent
 
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Sat Dec 29, 2007 9:39 pm



Quoting Blackbird (Thread starter):
Also the RM-8, was it modified at all from the baseline JT8D design?

The Volvo Flygmotor RM-8A or B(as was used in the Saab Viggen) was a standard JT8D but modified internally and an after burner was added.
It also features a different reverser.

Can't help you with the other questions.
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Jetlagged
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Sun Dec 30, 2007 12:48 am



Quoting Blackbird (Thread starter):
Okay... the JT8D's spool up time is around 6.5 to 8 seconds. What was the original J-52's spool-up rate? -- It seemed as if the A-6 intruders spooled up faster than that?

Also the RM-8, was it modified at all from the baseline JT8D design? Was it's spool-up rate different? Was it designed to fly faster?

The J52 was a turbojet, with one less compressor stage than a JT8D or an RM8. It might well have different dynamics. Also a civilian engine might be set up with a wider surge margin, and hence have its ACU set to spool up more slowly. A military variant could be allowed less surge margin and so accelerate faster. Whether the ACU was different I don't know.

The Volvo RM8B was certainly different to the JT8D as it apparently had three fan stages. The last question is rather obvious as clearly both the RM8A and RM8B were designed for a supersonic application.
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up

Sun Dec 30, 2007 7:11 am



Quoting Blackbird (Thread starter):

Also the RM-8, was it modified at all from the baseline JT8D design? Was it's spool-up rate different? Was it designed to fly faster?

"fly faster" is a bit of a misnomer. Even in supersonic flight, intake speed is slowed to subsonic. And that's as much as I know on the subject.

Quoting Blackbird (Thread starter):
(Assuming none of this is classified -- I doubt it would be since the A-6 is an outdated aircraft -- that is)

Yes and no. There are privately owned aircraft (museum pieces mostly) out there from the A-6 era that are not allowed to be made airworthy because they are too advanced. I'm sure there are plenty of third and fourth rate air forces that would (literally) kill to get a bunch of A-6s in service. And just because it's outdated doesn't make it totally ineffective against a first rate air power. Even in a best case scenario (from the first rate side) it would tie up resources.
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Blackbird
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Sun Dec 30, 2007 2:51 pm

Dear Jetlagged,

What's an ACU?

Regarding the design being used for supersonic operation? Does that mean like more air-cooling or something?


Andrea Kent
 
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Jetlagged
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Sun Dec 30, 2007 9:28 pm

ACU = Acceleration Control Unit. Part of the engine fuel control unit which limits fuel flow, so limiting engine acceleration to prevent surges.

Supersonic mods are mainly in the intake and exhaust nozzle area. The intake is designed to compress the air using shock waves and variable ramps and so reduce air velocity to subsonic. The additional air pressure is a useful bonus. The exhaust nozzle is variable area so the air can be expanded supersonically when necessary.
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Airgypsy
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Sun Dec 30, 2007 11:20 pm

ACU = Compressor Bleed Control for JT8s. The range of JT8s makes any general statement impossible. Straight 8s (1-17R) and the -200 series. The -17s bore almost no resemblance to the very popular early -7s. -200s came with a real fan and very advanced compressor control. Acceleration times are very dependant on the type and number of the bleed valves and the controller set up. I always gave a DC-9/MD-80 plenty of time to make up its mind when maintenance taxiing because unless you did a lot of checking first, you never knew what to expect when you pushed up the throttles.
6th stage, 8th stage, 13th stage bleed valve numbers changed with every effort to eliminate the chuffing and banging that was brought on by the EPA requirements to burn cleaner (particulate emissions). Unlike the GE compressor articulated stator vanes that provided a smooth transition from idle to power, the 8s shifted like a drag racer dropping the hammer when the bleed vaves closed (provided they didn't stick). Then there are the fuel control 3D cam issues with the 8s.
Besides - read closely and you'll find that acceleration limits are manufacturers "recommendations".
Military engines (like the Spey) are also saddled with governors and limiters that allow the operator to move the throttle without regard to overspeed or overtemp.
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Jetlagged
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Mon Dec 31, 2007 1:15 am



Quoting Airgypsy (Reply 6):
ACU = Compressor Bleed Control for JT8s

I was talking about the ACU within the fuel control unit, not surge bleed valves. Bleed valves certainly affect engine acceleration, but fuel control gives a more precise effect.

Quoting Airgypsy (Reply 6):
Military engines (like the Spey) are also saddled with governors and limiters that allow the operator to move the throttle without regard to overspeed or overtemp.

Not just a military thing. Rolls Royce typically added many more governors than were usual on a Pratt & Whitney engine of that era. That included civil Speys, Conways, RB211s.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
Blackbird
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Mon Dec 31, 2007 3:51 pm

Since the RM-8 was designed for high-speed flight in excess of Mach 2, conditions which would drive up the airplane's turbine inlet temperature, did it feature a modified turbine?

How quick were the RR-Ghost, RR-Avon, and RR-Spey spool-up rates if any of you guys know any of them by the way?


Starlionblue,

Quote:
There are privately owned aircraft (museum pieces mostly) out there from the A-6 era that are not allowed to be made airworthy because they are too advanced.

Like a B-58 or something?
 
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Mon Dec 31, 2007 6:02 pm



Quoting Airgypsy (Reply 6):
The -17s bore almost no resemblance to the very popular early -7s. -200s came with a real fan and very advanced compressor control.

What exactly do you mean by "Real Fan?" Are you saying the str8 8s were basically turbojets (in a sense)
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Blackbird
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Mon Dec 31, 2007 6:20 pm

So, even some older military turbojets in addition to having a fuel-control system designed with a narrower surge margin, and possibly more liberal use of bleed valves to allow faster spool-up, also were designed with governors and limiters to allow the pilot to achieve maximum performance out of the engine safely?


BR715-A1-30,

I would guess that he meant that the plane had a fan of more pronounced size than the earlier JT8D's, and a more substantial bypass-ratio.


Andrea Kent
 
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Mon Dec 31, 2007 11:56 pm



Quoting Blackbird (Reply 8):
Quote:
There are privately owned aircraft (museum pieces mostly) out there from the A-6 era that are not allowed to be made airworthy because they are too advanced.

Like a B-58 or something?

Some F-4s. Ok those are a bit younger but not much.
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Airgypsy
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Tue Jan 01, 2008 1:02 am

Straight 8s are "low" bypass engines. -200s are "medium" bypass engines. JT9s are "high" bypass. A J-75 turbojet is a no bypass.
Otherwise referred to as LRT = Little Round Things and BRT = Big Round Things.
 
Blackbird
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Tue Jan 01, 2008 2:24 am

Airgypsy,

I thought the cut-off for low-bypass ratio ended at 2.0 to 1... the JT8D-200 start at 1.76 to 1...


Andrea Kent
 
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Wed Jan 02, 2008 12:29 am

Is there really a definition of high and low bypass ratios? I don't know if you can really define it as a number.

Number aside, the straight 8's fan is like an early fan or fighter fan design. Multiple stages. An 8D-200 has a single stage fan and is more like a modern fan.
 
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RE: J-52 / JT8D / Swedish RM-8 Turbofan: Spool-up Time

Wed Jan 02, 2008 1:04 am



Quoting Airgypsy (Reply 12):
Otherwise referred to as LRT = Little Round Things and BRT = Big Round Things.

Personally I prefer big round things......................................................................

Easier to inspect.........
I hated trying to check in blades on a JT8. I much rather be standing in the inlet than bent up like a preztel in the inlet.

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